Composites are used in a wide variety of applications. For example, in aircraft construction, composites are used in increasing quantities to form the fuselage, wings, and tail section of conventional tube-and-wing aircraft. Composites may also be used in hybrid wing/body aircraft wherein the fuselage may be blended and integrated with the wings. Composite structures may include reinforcing elements to increase the overall strength and stiffness of the composite structure. For example, a composite panel may include a number of composite stiffeners such as frames and stringers that may be coupled to a composite skin member to increase the overall strength and stiffness of the composite panel. In one method of fabricating composite panels, a plurality of three-dimensional stringers may be formed as preforms of dry fabric material. The stringer preforms may be assembled to a relatively flat or curved skin preform. The skin and stringer preform assembly may be infused with resin and allowed to cure to form a final composite panel which may be used to form an airframe of an aircraft.
Conventional methods for forming three-dimensional stringer preforms include multiple hand operations to assemble, fold, and stitch the dry fabric. For example, conventional methods for forming stringer preforms require manually laying up or stacking individual pieces or plies of dry fabric into a ply stack. The method may additionally include manually positioning fabric ply doublers at one or more locations along a length of the ply stack. The ply stack may then be hand-folded to form a web which may be tack-stitched by hand and loaded into a stitching jig for inserting one or more web seams. Following the stitching of the web, the partially-formed stringer may be debulked, and manually-positioned in a preform assembly jig. After mounting in the assembly jig, the stringer flanges may be hand-formed, radius fillers may be installed, and tear straps may be manually applied over the flanges.
Unfortunately, the above-described preform forming process relies extensively on skilled touch labor which presents several drawbacks. For example, the large number of operations required for manually laying up, folding, and tack-stitching the dry fabric results in a lengthy cycle time for producing each stringer preform. For certain composite structures, a large quantity of stringer preforms may significantly add to the production schedule for the composite structure. Variations in stringer cross-section, variations in flange width, and localized ply doubler buildups along the stringer length add to the complexity and time required to fabricate each stringer preform. In addition, the high degree of accuracy required for forming each stringer preform may result in significant dimensional variation between stringer preforms and an excessively-high rejection rate requiring rework that may add to production costs.
As can be seen, there exists a need in the art for a system and method of forming a stringer preform that requires a minimal amount of recurring skilled touch labor. In addition, there exists a need in the art for a system and method of forming a stringer preform in a reduced amount of time and at reduced cost. Furthermore, there exists a need in the art for a system and method of forming stringer preforms with a high degree of accuracy and with minimal dimensional variation between stringer preforms.